Many kinds of semiconductor processing apparatus include a moving member. For example, a member may be provided to hold and translate the position of a semiconductor wafer for processing. An ion implanter, for example, is known from U.S. Pat. No. 5,898,179, in which the wafer is mounted upon a substrate holder in a process chamber. Attached to, or integral with, the substrate holder is an arm which extends through an aperture in the wall of the vacuum chamber. Mechanical scanning is effected by a scanning mechanism located outside the process chamber. The scanning mechanism is connected with the arm of the substrate holder and allows movement of the arm and hence the substrate holder relative to the process chamber.
Typically, the arm is substantially horizontal and is moved up and down in a reciprocating motion by a servo driven linear motor. Since the arm and substrate holder have significant combined mass, the effect of gravity on this mass makes it difficult for the servo and motor to provide precise velocity control throughout the upward and downward strokes.
In the ion implanter of U.S. Pat. No. 5,898,179, gravity provides a constant downward force on the arm and substrate holder.
Gravity is not the only possible cause of a constant force applied to a movable member. For example, when an elongate member extends into a vacuum chamber and is movable longitudinally, atmospheric pressure applies a constant inward force to the member.
In U.S. Pat. No. 4,726,689, a wafer holder is mounted in a vacuum chamber on a vertically aligned shaft which is longitudinally movable, the cross-section of the shaft is matched to the weight of the movable assembly, so that the downward force of gravity is counterbalanced by the upward force of atmospheric pressure.
In U.S. Pat. No. 6,172,372, the downward force of gravity on a vertically aligned shaft which penetrates opposite walls of a vacuum chamber is counterbalanced by a pneumatic actuator or hydraulic piston in which a constant internal pressure is maintained via a back relieving pressure regulator connected with a fluid supply or a constant pressure fluid supply pressurised by hydraulics or pneumatics. Counterbalancing is also provided to a system in which a longitudinally movable shaft protrudes through only one wall of a vacuum chamber.
It is an object of the present invention to provide a force compensator for applying a substantially constant force to a movable member over a displacement range which avoids the need to maintain a constant pressure in a pneumatic or hydraulic piston.